Process of using a cushion for laminating operations

ABSTRACT

Cellulosic paper sheets interleaved with sheets of a thermoplastic material having a glass transition point below about 100° C. and a melting point of about 125° C. or more provide a cushion for use in laminating operations. The thermoplastic sheets employed in the cushion have an individual thickness ranging from about 2 to 20 mils and a combined thickness of at least 5 mils. Such a cushion provides a long-term, reuseable cushion for laminating operations.

This is a continuation, of application Ser. No. 96,584, filed Nov. 21,1979 and now Pat. No. 4,264,404.

BACKGROUND OF THE INVENTION

Laminates have been employed as surface materials for application totables, vanities, vertical wall coverings, door coverings, and the likefor a number of years. Conventionally, these laminates are prepared byassembling, in superimposed relationship, a core layer which consists ofa plurality of kraft paper sheets impregnated with a thermosettingphenol-formaldehyde resin and a decorative sheet that is impregnatedwith a thermosetting resin which does not undergo noticeabledeterioration in color, such as darkening, upon any subsequentlaminating operations. Typical resins used in the decorative sheetinclude, but are not limited to, melamine-formaldehyde resins, epoxyresins, unsaturated polyester resins, and urea resins. The decorativesheet itself is conventionally an alpha-cellulose paper sheet which hasbeen dyed, pigmented or upon which there is imparted some design orpattern. Frequently, superimposed above the decorative sheet there isplaced an overlay sheet. The overlay sheet is a highly refinedalpha-cellulose paper sheet unpigmented, but impregnated with acolor-stable thermosetting resin.

The assembled laminate is heat and pressure consolidated underconventional conditions of pressure and temperature to produce a unitarylaminated product. During heat and pressure consolidation, thearrangement of plates, cushions, sheets and the laminate assembly mustbe such so as to most effectively and uniformly distribute the heat andpressure to the laminate. The cushion in such an arrangement acts toabsorb thermal shock and shear stress during pressing thereby aiding theuniform distribution of heat and pressure to the laminate. The cushionemployed in this arrangement has traditionally been comprised of aplurality of unimpregnated kraft paper sheets. Cushions of this type atthe top of the press pack have an average useful life of 5 pressingruns. When employed at the bottom of the press pack, their useful lifehas an upper limit of approximately 25 pressing runs. When used at thetop of the press pack, their useful life is somewhat shorter due toexcessive handling because, after each run, they are inspected fordamage. If no damage is discovered, the top-used cushions are eitherused again as the top cushion or reshuffled into the bottom of the presspack. Because of this, the cost of cushions to the laminating industryis presently approaching three million dollars annually and escalating.Additionally, the conventional kraft cushions currently employed oftimesresult in a peripheral gloss around the edges of the laminate, calledthe picture frame effect, requiring the edges to be trimmed off of thelaminates so produced before they are sold. This additional trimmingstep contributes to the cost of laminate manufacture especially when apaper texturizer has been employed. Occasionally, water marks will alsoresult from the use of these conventional cushions in that theuniformity of the heat and pressure fluctuates when they have been useda number of times.

Thus, there exists the need for a cushion that exhibits a greater usefullife and yet continues to effectively and uniformly distribute heat andpressure to the decorative laminate assembly. Accordingly, the provisionfor a more economical and efficient cushion would fulfill a long-feltneed and constitute a significant advance in the art.

SUMMARY OF THE INVENTION

The present invention provides for a novel cushion for use in laminatingoperations and a process of producing a laminate wherein such a cushionis employed. The cushion enables a uniform distribution of heat andpressure to the laminate, eliminates the peripheral gloss and watermarkings of the laminates prepared, and has a useful life of over 100pressings. Surprisingly, the instant cushion also aids in theelimination of flash, dirt and foreign particles which are attracted tothe thermoplastic material and are transferred during subsequentpressings to the laminate. The cushion is easily handleable and providesfor a cost reduction of up to about 80% over the conventional cushions.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the instant invention, there is provided a laminatingcushion comprising at least two cellulosic paper sheets and at least onethermoplastic sheet having a glass transition point below about 100° C.and a melting point of about 125° C. or more wherein the thermoplasticsheets have an individual thickness ranging from about 2 to 20 mils anda combined thickness greater than 5 mils and wherein the individualthermoplastic sheets are interleaved between the cellulosic papersheets. Additionally, there is provided a process for producing alaminate comprising placing an assembly comprising, in superimposedrelationship, (1) a caul plate of a laminate press, (2) a cushion, (3) arelease sheet, (4) a thermosetting resin impregnated laminate assemblyand (5) a press plate within a laminate assembly press and thereaftereffecting consolidation of the laminate assembly by applying sufficientheat and pressure to thermoset the resins impregnating the laminateassembly wherein the cushion employed comprises at least two cellulosicpaper sheets and at least one thermoplastic sheet wherein thethermoplastic sheets have an individual thickness ranging from about 2to 20 mils and a combined thickness greater than about 5 mils andwherein the individual thermoplastic sheets are interleaved between thecellulosic paper sheets.

Suitable cellulosic paper sheets include, but are not limited to,cellulosic paper sheets prepared from kraft, acid sulfite, oxygen, sodaand neutral sulfite pulping processes. Preferably, they are of the typeconventionally employed in laminate cushions, i.e., a kraft paper sheetof about a 30 to 130 pound basis weight per 3000 square foot reamavailable commercially in plentiful supply, but may also includealpha-cellulose sheets and especially those alpha-cellulose sheets whichwere formed into decor sheets but are not employed as such because thedecorative design thereon becomes obsolete.

Suitable thermoplastic material includes sheets of polypropylene,polycarbonate, polystyrene and the like. These thermoplastic sheetsexhibit a glass transition point below about 100° C. and a melting pointof about 125° C. or more. These thermoplastic sheets should have athickness of about 2 to 20 mils. The preferred mode of the instantinvention is to employ 3 or more thermoplastic sheets within the cushionwith each individual sheet having a thickness of from about 3 to 10 milsso that the combined thickness of the thermoplastic sheets in thecushion is greater than about 9 mils. Preferably, no more than 10cellulosic paper sheets and 9 thermoplastic sheets are interleaved in acushion. However, in special applications, it may become necessary touse about 20, 30, or 40 or more cellulosic paper sheets interleaved withthermoplastic sheets. It has been found that by placing thermoplasticsheets together, rather than interleaving them between cellulosic papersheets, so that their adjacent thickness exceeds 20 mils creates acushion that is too fluid and thus does not uniformly distribute theexerted pressure to the thermosetting resin impregnated laminateassembly. However, placing thermoplastic sheets together so that theiradjacent thickness is less than or equal to 20 mils does not severelyeffect the laminating process. However, when these thermoplastic sheetsare used alone, static electricity is generated and attracts flash andother laminate debris thereby damaging laminates produced therefrom.Thus, interleaving of thermoplastic sheets and cellulosic paper sheetsremains the preferred embodiment.

The laminate cushion must have at least two cellulosic paper sheets andat least one thermoplastic sheet. Generally, the laminate cushion willconsist of one less sheet of thermoplastic material sheet than there aresheets of cellulosic paper. In the preferred configuration there arefour sheets of cellulosic paper having three sheets of thermoplasticmaterial interleaved between the cellulosic paper sheets. Although thelaminate cushion itself need not be prepressed before use in thelaminating process, because pressing thereof occurs during laminateproduction, it is preferred that such prepressing be conducted so as tofacilitate handling of the cushion per se.

In some instances, it may be desirable to lightly bond the bottomcushion to the caul plate. In these instances, a hot melt adhesive isplaced between the caul plate and the bottom cushion so as to absolutelyassure that no accumulation of foreign particles occurs between the caulplate and bottom cushion. In the preferred instances, polyethylene isemployed as the hot melt adhesive since it can easily be stripped fromthe caul plate when the cushion subsequently becomes physically damagedor of no further use.

To manufacture high-pressure laminates, there is exerted upon theassembly a pressure of from about 800 to 1600 psi at temperaturesranging from about 120° C. to 180° C. for approximately 30 minutes.Low-pressure laminates are prepared under substantially identicalconditions except that a pressure from about 200 to 600 psi is exertedand the time of pressing is much shorter. The press plates employedduring these pressing operations range from polished stainless steelplates to textured plates which impart a textured or embossed surface tothe laminate.

The following specific examples illustrate certain aspects of thepresent invention and, more particularly, point out advantages providedby the present invention. However, the examples are set forth forillustration only and are not to be construed as limitations on thepresent invention except as is set forth in the appended claims.

COMPARATIVE EXAMPLE A

Placed in superimposed relationship upon the caul plate of a laminateassembly press are five unimpregnated kraft paper sheets functioning asthe bottom cushion; a first coated release sheet; a first stainlesssteel press plate; a first texturizer sheet; a first laminate assembly;a second coated release sheet; a second laminate assembly; a secondtexturizer sheet; a second stainless steel press plate; a third coatedrelease sheet; five unimpregnated kraft paper sheets functioning as thetop cushion; and the press platen. The laminate assembly consists of acore layer of phenol-formaldehyde resin impregnated kraft paper sheets,a decor sheet impregnated with a melamine-formaldehyde thermosettingresin and an alpha-cellulose overlay sheet impregnated with amelamine-formaldehyde thermosetting resin. The entire assembly issubjected to conventional high-pressure lamination and allowed to cool.Inspection reveals peripheral gloss of the laminates and an earlyindication of the loss of cushionability of the top and the bottomcushion.

COMPARATIVE EXAMPLE B

Following the procedure of Comparative Example A, the same top andbottom cushions are employed in a series of laminations replacing therelease sheets, texturizer sheets and laminates after each operation.After 18 laminating runs, the top and bottom sheets show signs of wearand requires replacement and the laminates produced in the last runsexhibit water marks from non-uniform heat and pressure distributionduring pressing.

EXAMPLE 1

Following the procedure of Comparative Example A, there is employed asthe top and bottom cushion a cushion comprising, in superimposedrelationship, a first sheet of kraft paper sheet; a first sheet ofpolypropylene; a second sheet of kraft paper sheet; a second sheet ofpolypropylene; a third sheet of kraft paper sheet; a third sheet ofpolypropylene and a fourth sheet of kraft paper. The polypropylenesheets employed individually have a thickness of 3 mils. Inspectionafter pressing shows no wear on either cushion and no sign of peripheralgloss on the laminates produced.

EXAMPLE 2

The procedure of Example 1 is followed in a series of laminate pressingsemploying the same cushions in each pressing. After 134 pressings, thetop and bottom cushions still show no sign of deterioration which wouldnecessitate their replacement. The laminates so produced aresubstantially free of water marks and peripheral gloss.

COMPARATIVE EXAMPLE C

Following the procedure of Comparative Example A, there is employed asthe top and bottom cushions five sheets of polypropylene each. Thepolypropylene sheets employed are individually 7 mils in thickness.Inspection reveals that static attributable to the polypropylene sheetshas attracted foreign particles that mar the laminate surface.

EXAMPLE 3

When the procedure of Example 1 is followed in every material detailexcept that there is employed as the top and bottom cushion a cushioncomprising, in superimposed relationship, a first kraft paper sheet; afirst polypropylene sheet; and a second kraft paper sheet, and thepolypropylene sheet employed has a thickness of 18 mils, inspectionreveals substantially equivalent results to that obtained in Example 1.

EXAMPLE 4

When the procedure of Example 3 is followed in a series of laminationsemploying the same cushions, results substantially equivalent to Example2 are obtained.

EXAMPLE 5

When the procedure of Example 1 is followed in every material detailexcept that there is employed as the top and bottom cushion a cushioncomprising eight kraft paper sheets with seven polypropylene sheets eachindividually 4 mils in thickness interleaved between the kraft papersheets substantially equivalent results are obtained to those reportedin Example 1.

EXAMPLE 6

When a series of laminations employing the cushions of Example 5 arerun, results substantially equivalent to Example 2 are obtained.

EXAMPLE 7

When the procedure of Example 1 is followed in every material detailexcept that there is employed as the top and bottom cushion a cushioncomprising two acid sulfite paper sheets with one 10-mil polycarbonatesheet interleaved between the paper sheets substantially equivalentresults are obtained to those reported in Example 1.

EXAMPLE 8

When a series of laminations employing the cushions of Example 7 arerun, results substantially equivalent to Example 2 are obtained.

EXAMPLE 9

When the procedure of Example 1 is followed in every material detailexcept that there is employed as the top and bottom cushions a cushioncomprising four obsolete alpha-cellulose decor paper sheets with three4-mil polypropylene sheets interleaved between the obsoletealpha-cellulose decor paper sheets, substantially equivalent results areobtained to those reported in Example 1.

EXAMPLE 10

When the procedure of Example 1 is followed in every material detailexcept that there is employed as the top and bottom cushions a cushioncomprising six kraft paper sheets with five 5-mil polystyrene sheetsinterleaved between the kraft paper sheets, substantially equivalentresults are obtained to those reported in Example 1.

What is claimed is:
 1. A process for producing a heat and pressureconsolidated laminate which comprises heat and pressure consolidating alaminate assembly while employing, as a pressure cushion, at least twocellulosic sheets and at least one thermoplastic sheet having a glasstransition point below 100° C. and a melting point of about 125° C. ormore wherein the thermoplastic sheet or sheets have an individualthickness ranging from about 2 to 20 mils and a combined thicknessgreater than about 5 mils and wherein the individual thermoplasticsheets are interleaved between the cellulosic paper sheets.